1. Field of the Invention
The present invention relates generally to a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to a DPCH (Dedicated Physical Channel) multiplexing apparatus and method for performing outer loop power control by properly maintaining a target SIR (Signal-to-Interference Ratio).
2. Description of the Related Art
In general, a channel structure of a UMTS (Universal Mobile Terrestrial System) CDMA mobile communication system is classified into a physical channel, a transport channel and a logical channel. The physical channel is divided into a downlink physical channel and an uplink physical channel according to its data transmission direction. Further, the downlink physical channel is divided into a physical downlink shared channel (PDSCH) and a downlink dedicated physical channel (DPCH), which will be described with reference to FIG. 1.
FIG. 1 illustrates a structure of a downlink dedicated physical channel in a mobile communication system. Referring to FIG. 1, each frame of the downlink dedicated physical channel is comprised of 15 slots Slot#0-Slot#14. Each slot is comprised of dedicated physical data channels (DPDCHs) for transmitting upper layer data from a Node B to a UE (User Equipment), and dedicated physical control channels (DPCCHs) for transmitting a physical layer control signal. The dedicated physical control channel DPCCH is comprised of a TPC (Transport Power Control) symbol for controlling transmission power of the UE, a TFCI (Transport Format Combination Indicator) symbol, and a pilot symbol. As illustrated in FIG. 1, each of the slots Slot#1-Slot#14 constituting one frame of the downlink dedicated physical channel is comprised of 2560 chips. in FIG. 1, a first data symbol Data1 and a second data symbol Data2 represent upper layer data transmitted from the Node B to the UE over the dedicated physical data channel DPDCH, and the TPC symbol represents information for controlling transmission power of the UE by the Node B. Further, the TFCI symbol indicates a transport format combination (TFC) used for a downlink channel transmitted for a current one frame (=10 ms). Finally, the pilot symbol represents a criterion for controlling transmission power of the dedicated physical channel by the UE. Here, information included in the TFCI can be classified into a dynamic part and a semi-static part. The dynamic part includes TBS (Transport Block Size) information and TBSS (Transport Block Set Size) information. The semi-static part includes TTI (Transmission Time Interval) information, channel coding scheme information, coding rate information, static rate matching information, and CRC (Cyclic Redundancy Check) size information. Therefore, the TFCI indicates the number of transport blocks (TB) in a channel transmitted for one frame, and assigns unique numbers to TPCs used in each of the transport blocks.
FIG. 2 illustrates a structure of an uplink dedicated physical channel in a mobile communication system. Referring to FIG. 2, like the downlink dedicated physical channel, the uplink dedicated physical channel is comprised of 15 slots Slot#1-Slot#14. The uplink dedicated physical channel has an uplink dedicated physical data channel (DPDCH) and an uplink dedicated physical control channel (DPCCH). Each of the slots Slot#0-Slot#14 constituting one frame of the uplink dedicated physical data channel DPDCH transmits upper layer data from the UE to the Node B.
Meanwhile, each of the slots Slot#0-Slot#14, which constitutes one frame of the uplink dedicated physical control channel is comprised of (i) a pilot symbol used as a channel estimation signal when demodulating data transmitted from the UE to the Node B, (ii) a TFCI symbol indicating a transport format combination (TFC) of channels transmitted for a current frame, (iii) an FBI (FeedBack Information) symbol for transmitting feedback information when transmission diversity is used, and (iv) a TPC symbol for controlling transmission power of the downlink channels.
Transmission power of the downlink/uplink dedicated physical channels shown in FIGS. 1 and 2 is controlled by a high-speed power control method such as a closed-loop power control method or an outer loop power control method. Herein, the outer loop power control will be described.
The outer loop power control method compares a target SIR required in the high-speed power control method with an actual SIR of the channel, for both the downlink channel and the uplink channel, and controls the transmission power by resetting a threshold for the closed-loop power control based on the comparison result between the target SIR and the actual SIR. In general, it is important for the power control method to maintain a bit error rate (BER) or a block error rate (BLER) in order to satisfy required communication performance. The outer loop power control method maintains the BER or the BLER at a required level by continuously resetting a threshold for maintaining the BER or the BLER. The UE and the Node B may measure the BER or the BLER through CRC error detection by analyzing CRC bits included in the received dedicated physical data channel.
FIG. 5 illustrates a structure of a physical downlink shared channel (PDSCH) in a mobile communication system. Referring to FIG. 5, a 10 ms-frame of the physical downlink shared channel is comprised of 15 slots Slot#0-Slot#14. Since the UMTS system has a chip rate of 3.84 Mcps, each of the slots is comprised of 2560 chips.
The physical downlink shared channel transmits upper layer data from the Node B to the UE in association with the dedicated physical channel, for power control and transport format combination indication. The physical downlink shared channel is shared by a plurality of UEs on a time division basis to efficiently transmit a large amount of packet data to the UEs. In order for the UE to use the physical downlink shared channel, separate dedicated physical channels between the UE and the Node B, (namely, the downlink dedicated physical channel and the uplink dedicated physical channel associated (or interlocked) with the physical downlink shared channel) should be maintained. Therefore, in order for the UE to use the physical downlink shared channel, it should separately establish the downlink and uplink dedicated physical channels. For example, if N UEs use the physical downlink shared channel, N downlink and N uplink dedicated physical channels (i.e., one such dedicated channel to each UE) are established so that the N UEs share the physical downlink shared channel on a time division basis. Meanwhile, the physical downlink shared channel is a physically established channel so as to transmit a large amount of packet data, while the dedicated physical channel is physically established to transmit a relatively small amount of control data and retransmission-related data, compared with the physical downlink shared channel. A detailed description of this will be made herein below.
A TFCI bit TFCIDPCH transmitted over the dedicated downlink physical channel has information indicating a transport format of the physical downlink shared channel. Therefore, the downlink TFCI indicates a UE to which packet data was transmitted over the physical downlink shared channel after a lapse of a predetermined time from a given time point. The UE can recognize whether there is physical downlink shared channel data to receive, by continuously analyzing the downlink dedicated physical channel received. Therefore, when the TFCI received by the UE indicates that there exists data to receive in the physical downlink shared channel of the next frame, the UE receives the data transmitted by the Node B by demodulating and decoding a signal received over the physical downlink shared channel at the next frame. During the data transmission over the dedicated physical channel, transmission power is controlled using the outer loop power control, a description of which will be separately made for normal transmission and gated transmission.
When the uplink or downlink channel has no transport channel data during normal transmission, i.e., normal data transmission, CRC bits are transmitted over the dedicated physical channel for the outer loop power control. However, if only the CRC bits are transmitted or repeated for the outer loop power control while there is no transport channel data, a combining gain will occur at the receiver, causing a decrease in a target SIR. Therefore, when there is transport channel data generated later, the BLER becomes high until the target SIR is recovered, because of the decrease in the target SIR due to transmission of only the CRC bits during non-existence of the transport channel data.
In addition, even when the outer loop power control is applied to the gated transmission, in order to perform outer loop power control while gating a dedicated physical control channel during data communication where a dedicated channel (DCH) is interlocked with a downlink shared channel (DSCH), it is necessary to measure the BER or BLER through CRC error detection. A detailed description of this will be made herein below.
Herein, a state where the downlink shared channel and the dedicated channel are established will be defined as a “DSCH/DCH state”. In the DSCH/DCH state, a UE in data communication should transmit/receive a downlink dedicated channel signal and an uplink dedicated channel signal interlocked with the downlink shared channel, in order to maintain a proper channel state through power control for a waiting time. Continuously transmitting/receiving the downlink and uplink dedicated channel signals in order to maintain the channels wastes battery power of the UE and increases interference to the downlink and the uplink, thus limiting the number of UEs that can share the downlink shared channel.
To solve this problem, the UMTS channel scheme performs DPCCH gating for efficient radio channel management by optionally reducing the number of slot signals (15 slots/frame) transmitted for every 10 ms-frame over the dedicated physical control channel in a state where the dedicated physical data channel has no information data (including CRC bits and tail bits). That is, since that the dedicated physical control channel is subject to gating means that there is no user data transmitted over the dedicated physical data channel, a length of the user data becomes zero (0). A start and end of the DPCCH (Dedicated Physical Control Channel) gating operation can be performed through either a control message from an upper layer, i.e., a Layer 3, or a TFCI bit. As a result, it is possible to secure efficient utilization of radio resources and reduce battery consumption by the UE, by reducing an amount of radio channel resources required in maintaining the dedicated physical channel for the period where no user data is transmitted over the physical channel due to the DPCCH gating operation.
In the DPCCH gating mode, there is no user data (including CRC bits and tail bits), so data transmission over the dedicated physical data channel is suspended. Therefore, a process for multiplexing the downlink or uplink dedicated physical data channel is not required. However, in order to perform outer loop power control even while performing the DPCCH gating, it is necessary to measure the BER or BLER through CRC error detection. Therefore, even though there is no user data to transmit during the DPCCH gating, the dedicated physical data channel including the CRC should be transmitted.
As described above, in the gated transmission mode, only the CRC is repeatedly transmitted over the dedicated physical data channel, so combining occurs at the receiver, causing a decrease in the target SIR. As a result, when transmitting transport channel data after the end of the DPCCH gating, the BLER becomes high until the target SIR is recovered, because of the decrease in the target SIR due to the DPCCH gating, thus making it difficult to secure reliable outer loop power control.
Specifically, a DPCH (Dedicated Physical Channel) multiplexing method performs rate matching using Equation (1) defined in the 3GPP (3rd Generation Partnership Project) standard (see 3GPP TS25.212 V3.4.0: Multiplexing and Channel Coding).
                                          Z                          0              ,              j                                =          0                ⁢                                  ⁢                              Z                          i              ,              j                                =                      ⌊                                          (                                                      (                                                                  ∑                                                  m                          =                          1                                                i                                            ⁢                                                                        RM                          m                                                ×                                                  N                                                      m                            ,                            j                                                                                                                )                                    ×                                      N                                          data                      ,                      j                                                                                                                    ∑                                      m                    =                    1                                    i                                ⁢                                                      RM                    m                                    ×                                      N                                          m                      ,                      j                                                                                            ⌋                          ⁢                                  ⁢                                            for              ⁢                                                          ⁢              all              ⁢                                                          ⁢              i                        =            1                    ,          …          ⁢                                          ,          l                ⁢                                  ⁢                              Δ            ⁢                                                  ⁢                          N                              i                ,                j                                              =                                    Z                              i                ,                m                                      -                          Z                                                i                  -                  1                                ,                j                                      -                          N                              i                ,                j                                                    ⁢                                  ⁢                                            for              ⁢                                                          ⁢              all              ⁢                                                          ⁢              i                        =            1                    ,          …          ⁢                                          ,          l                                    Equation        ⁢                                  ⁢                  (          1          )                    
In Equation (1), Nl,j for the uplink represents the number of bits included in one radio frame of an ith transport channel of a transport format combination (TFC) j before rate matching and for the downlink represents a multiple of ⅛, an intermediate parameter used in the rate matching process. Further, Ndata,j represents the total number of bits filled in CCTrCH (Coded Composite Transport Channel) included in one radio frame of the transport format combination j, RMl represents a rate matching constant of an ith transport channel, and Zi,j represents an intermediate rate matching parameter. In addition, for the uplink, ΔNl,j represents a final target value in rate matching. If the ΔNl,j is a positive number, it represents the number of bits repeated within one radio frame of the ith transport channel of the transport format combination j, and if the ΔNl,j is a negative number, it represents the number of punctured bits. However, for the downlink, the ΔNi,j is used as an intermediate parameter, a value of which is a multiple of ⅛, and 1 represents the number of transport channels included in the CCTrCH.
In the uplink channel, since transmission data is subject to rate matching after being segmented in a radio frame unit, the number ΔNi,j of repeated or punctured bits of the radio frames is calculated in accordance with Equation (1) based on Nl,j and Ndata,j, and the rate matching is performed in the process disclosed in 3GPP TS25.212.
However, in the downlink channel, since the transmission data is subject to rate matching in a TTI unit before being segmented in a radio frame unit, the rate matching is performed based on Ni,lTTI unlike in the uplink channel, and this method is disclosed in 3GPP TS25.212. The Ni,lTTI is a parameter used only in the downlink, and represents the number of bits included in one TTI for the case of a transport format l in the ith transport channel before rate matching. In the case of the downlink channel, the positions of the transport channels in the radio frame can be either fixed regardless of the transport format combination or varied according to the transport format combination. The intermediate parameters Nl,j and ΔNi,j used in Equation (1) have a different calculation method and also have a different rate matching process according to circumstances. In the case of the downlink channel, since Ndata,j does not depend on j, it is replaced with Ndata,* in Equation (1).
In the downlink channel, if the transport channels have the fixed positions, N1,j does not depend upon j. Therefore, it is replaced with Nl,*. After Ni,* is calculated in accordance with Equation (2) below, ΔNl,* is calculated in accordance with Equation (1) using the values of Nl,* and the Ndata,*. From the calculated ΔNi,*, a rate matching target value ΔNi,lTTI is calculated in a TTI unit of a transport channel i with a transport format l by the process defined in 3GPP TS25.212. If the ΔNi,lTTI is a positive number, it represents the number of bits repeated in each TTI of the transport channel i with the transport format l. However, if the ΔNi,lTTI is a negative number, it represents the number of punctured bits.
                              N                      i            ,            *                          =                              1                          F              i                                ×                      (                                          max                                  l                  ∈                                      TFS                    ⁡                                          (                      i                      )                                                                                  ⁢                              N                                  i                  ,                  l                                TTI                                      )                                              Equation        ⁢                                  ⁢                  (          2          )                    
In Equation (2), Fl indicates the number of radio frames included in one TTI of the transport channel i, and TFS(i) indicates a set of a transport format index l for the transport channel i.
In the downlink channel, if the transport channels have variable positions according to the transport format combination, Ni,j is calculated in accordance with Equation (3), and then, ΔNi,j is calculated in accordance with Equation (1) using the Ni,j and the Ndata,*. The rate matching target value ΔNi,lTTI is calculated in a TTI unit of the transport channel i with the transport format l based on the calculated ΔNi,j and the process defined in 3GPP TS25.212.
                              N                      i            ,            j                          =                              1                          F              i                                ×                      N                          i              ,                                                TF                  i                                ⁡                                  (                  j                  )                                                      TTI                                              Equation        ⁢                                  ⁢                  (          3          )                    
In Equation (3), TFi(j) represents a transport format of the transport channel i for the transport format combination j.
Therefore, if channel coding is performing by transmitting only the CRC and/or the tail bit required in measuring the BER or BLER for outer loop power control in a state where there is no user data, the rate matching is performed in accordance with Equations (1) to (3) and the process defined in 3GPP TS25.212, thus the number of bits repeated in rate matching after channel coding is larger than when the transport channel data and the CRC are transmitted together. Therefore, when the user data is normally transmitted over the dedicated physical data channel after the end of the DPCCH gating, the target SIR is set to a relatively low value due to the outer loop power control performed by transmitting only the CRC, so that it is not possible to efficiently perform the high-speed power control at an initial power control stage. This problem commonly occurs when performing the outer loop power control by transmitting only the CRC, regardless of whether the gating is applied.